This invention relates to an air flow sensor with a temperature dependent resistor device such as thermistor and a unique circuit for calculating the rate of heat loss by the temperature dependent resistor device.
Hot body anemometry is a common method used in air flow measurement. A temperature dependent resistor device such as a thermistor or a hot wire is heated up to a temperature higher than the ambient temperature and the rate of heat loss is measured. The air flow over the hot device causes transfer of heat from the device to the air flowing over it. The rate of heat loss is directly related to the temperature difference between the hot body and the ambient air, the air flow velocity, the density of the air, the thermal properties of the hot body, and its geometry.
If the rate of heat transfer and the ambient air temperature can be measured, the air flow rate over the hot body can be computed in accordance with King""s law, assuming that all of the other parameters are constant. The geometry and the material thermal properties such as the specific heat and the surface area of the hot body are constant. The ambient air temperature can be measured using sensors such as a thermistor, an RTD, or a thermocouple.
There are several methods used for determining the rate of heat loss from the temperature dependent resistor device. One common approach is to measure the power required to keep the device at a constant temperature, which is a measurement of its rate of heat loss. This method requires measuring the voltage across the device at a known resistance. An analog to digital converter is usually required to measure the voltage. Since the power drop across the device is proportional to the square of the voltage, a very precise analog to digital converter is required to measure the voltage to achieve an acceptable power measurement. Since power dissipation is proportional to the square of the voltage, an error in the voltage measurement results in a magnified error in the calculation of the heat loss rate. For example, a voltage measurement error of 5% leads to a 10% error in the heat loss rate measurement calculation.
In another approach, the voltage across the device with a constant current running through it is measured. In this example, the power dissipated can be measured as the product of the voltage and current through the device. However, the temperature of the device, which is an important parameter in determining air flow, varies with the power dissipated and hence the device temperature must be determined independently. Moreover, even this method requires an expensive and precise analog to digital converter.
It is therefore an object of this invention to provide an air flow sensor which does not require an analog to digital converter to determine the heat loss rate of the temperature dependent resistor device.
It is a further object of this invention to provide such an air flow sensor which does not require a separate sensor for measuring the temperature of the temperature dependent resistor device.
It is a further object of this invention to provide such a sensor which more accurately determines the heat loss rate of the temperature dependent resistor device.
It is a further object of this invention to provide such an air flow sensor which is easy to design and implement.
It is a further object of this invention to provide such an air flow sensor which allows the ambient air temperature to be measured by the temperature dependent resistor device itself, eliminating the need for a separate sensor.
It is a further object of this invention to provide such an air flow sensor which is voltage source independent.
This invention results from the realization that the rate of heat loss of a temperature dependent resistor device such as a thermistor, a variable required in order to calculate the air flow over the thermistor, can be measured without using a costly analog to digital converter which results in erroneous heat loss rate measurements, but instead by timing how long the thermistor takes to cool from a high temperature where the resistance is low to a lower temperature where the resistance is higher using one circuit which balances at the lower resistance value and another circuit which balances at the higher resistance value and comparators which determine when each circuit balances and thus when the thermistor is at the high temperature and then, after cooling, reaches the lower temperature. By monitoring the output of each comparator, the time it takes the thermistor to cool from the higher temperature to the lower temperature can be measured and the rate of heat loss of the thermistor calculated using as input the high and low temperatures and the cooling time.
This invention features an air flow sensor comprising a temperature dependent resistor device and a first circuit for applying a voltage to the temperature dependent resistor device until it reaches a first temperature. The first circuit typically includes a first reference resistance leg, a first variable resistance leg including the temperature dependent resistor device, and a first comparator connected to both legs for determining when the temperature dependent resistor device reaches the first temperature. There is also a second circuit including a second reference resistance leg, a second variable resistance leg including the temperature dependent resistor device, and a second comparator connected to both legs for determining when the temperature dependent resistor device reaches a second temperature.
A processor is connected to both the first and second comparators and programmed to time the period of time it takes the temperature dependent resistor device to change from the first temperature to the second temperature to determine the heat loss rate of the temperature dependent resistor device.
The temperature dependent resistor device may be a thermistor. The first variable resistance leg may include a low impedance resistor connected in series with the temperature dependent resistor device. The first reference resistance leg typically includes a plurality of resistors connected in series and the first reference resistance leg is connected in parallel with the first variable resistance leg.
The second reference resistance leg typically includes a plurality of resistors connected in series. The second variable resistance leg may include a high impedance resistor connected in series with the temperature dependent resistor device and the second reference resistance leg is connected in parallel with the second variable resistance leg. A first switch is connected between a voltage source and the first circuit and the processor is programmed to close the first switch until the temperature dependent resistor device reaches the first temperature and to then open the first switch. A second switch is connected between a voltage source and the second circuit and the processor is further programmed to close the second switch after the temperature dependent resistor device reaches the first temperature.
The air flow sensor may further include an ambient temperature sensing circuit including the temperature dependent resistor device. The ambient temperature sensing circuit may include a reference resistor and a capacitor connected in series with the temperature dependent resistor device. The processor is connected on a first line between the reference resistor and the capacitor and on a second line between the capacitor and the temperature dependent resistor device. The processor is programmed to apply a voltage on the first line and to detect the voltage on the second line until it reaches a predetermined level and to then apply a voltage on the second line and to detect the voltage on the first line until it reaches the predetermined level.
An air flow sensor in accordance with this invention includes a temperature dependent resistor device; means for applying a voltage to the temperature dependent resistor device until it reaches a first temperature; means for determining when the temperature dependent resistor device then cools to a second, temperature; and means for timing the period of time it takes the temperature dependent device to change from the first temperature to the second temperature to determine the heat loss rate of the temperature dependent resistor device.
The means for applying a voltage may include a first switch connected between a voltage source and a first circuit which includes a comparator connected to a first reference resistance leg and a first variable resistance leg including the temperature dependent resistor device, the comparator providing an output signal when the resistance of the temperature dependent resistor device causes the first circuit to balance. The means for determining may include a second switch between a voltage source and a second circuit which includes a comparator connected to a second reference resistance leg and a second variable resistance leg including the temperature dependent resistor device, the comparator providing an output signal when the resistance of the temperature dependent resistor device causes the second circuit to balance.
A method of determining the heat transfer rate of a temperature dependent resistor device in accordance with this invention includes the steps of heating the temperature dependent resistor device to a first temperature; allowing the temperature dependent resistor device to cool to a second temperature; measuring the period of time it takes for the temperature dependent resistor device to cool to the second temperature; and calculating the rate of heat transfer of the temperature dependent resistor device based on the measured period of time. The step of heating typically includes applying a first voltage across the temperature dependent resistor device until it reaches a first resistance value. The step of allowing the temperature dependent resistor device to cool includes applying a second voltage across the temperature dependent resistor device until it reaches a second resistance value. The step of measuring may include monitoring when the temperature dependent resistor device reaches the first resistance value and timing the time period it takes to reach the second resistance value.